Process for transmitting or storing over disturbed channels a digitalized, encoded audio signal composed of a series of information blocks

ABSTRACT

Digitalized encoded audio signals for transmission over disturbed channels are composed of a series of information blocks made of information units containing various types of information, such as control information, scale factor information and information on encoded scanning values derived from partial-band and/or transformation coding at the source. The units of information concerning the encoded scanning values are allocated to a predetermined spectral structure. Before its transmission or storage, the encoded audio signal is subjected to a channel encoding dependent on the desired error protection. When errors are recognized, they are corrected, and when the errors cannot be corrected, they are concealed. For the channel encoding of the units of information concerning the encoded scanning values, a variable bit error protection is provided depending on the allocation of the individual information unit to a determined spectral structure, i.e., units of information concerning the encoded scanning values of lower frequency audio signal fractions are given a higher bit error protection than units of information concerning the encoded scanning values of higher frequency audio signal fractions. The variable error protection to be selected for the individual information units is further determined depending on the admissible duration and frequency, according to subjective criteria, of the applied error concealment.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of transmitting or storing a digitizedencoded audio signal composed of a sequence of information blocks overinterference affected channels, wherein the individual informationblocks are composed of information units containing differentinformations, such as control informations, scale factor informations aswell as informations about the encoded sampled values derived from asub-band and/or transformation source coding process, with the units forthe informations being associated with respect to the encoded sampledvalues with a certain spectral structure and wherein the encoded audiosignal is subjected to channel coding that is dimensioned according tothe desired error protection, with an error correction being effected inthe case of the detection of an error and, if such errors are notcorrectable, the errors being concealed. Such a method is disclosed inInternational Publication WO A, 92 179 48.

2. Description of the Related Art

For the transmission of digitally coded audio signals over radiochannels or other interference affected channels it is desired to have arelatively high bit error protection at the transmitting end in order toensure reception under poor transmission conditions as well, forexample, when there is shading in mobile reception. This requires acorrespondingly high redundancy in the transmitted binary positions ofthe digitally coded signal which is connected with high data rates. Toreduce the data rate, bit saving source codes based on a reduction ofredundance and irrelevance are usually employed before the channelcoding. In such bit saving source coding methods, the digitally sampledaudio signal is converted into sub-band sampled values by using a filterbank and/or into spectral sampled values by using a transformation intothe frequency domain. In addition to the sampled values, additional andcontrol informations, such as, for example, scale factors, scale factorselection informations and bit assignment informations are transmitted,that is, successive information units of different content must betransmitted, with a sequence of such information units being combinedinto an information block (also called "information frame"). Bit errorsat individual binary positions within an information block produce adifferent, subjective noise effect for the listener. For example, theinterference in a bit assignment in the radio channel may temporarilylead to a complete absence on the playback side and thus to muting ofthe audio signal for a certain duration. These control informations musttherefore be protected to a considerably greater extent than the encodedsampled values. In connection with the error protection of the encodedsampled values, it applies that the listener has difficulties discerninga bit error in a lower value binary position of the encoded sampledvalue and sometimes is unable to discern it at all.

DE 3,805,169.C2 discloses the provision of a variable bit errorprotection which is adapted to the varying significance of theindividual binary positions of each information block. In the case ofincreasingly poorer transmission conditions, the less important binarypositions of the information block are interfered with first since thesebinary positions receive less error protection than the higher valuedbinary positions. With increasingly poorer transmission conditions, theinterference progresses to increasingly more important binary positions,with the subjective noise effect being describable as "discernible","slightly annoying" up to "unpleasantly annoying". In this connection itis assumed that it is possible only to correct the error in the receivedencoded audio signal data not to conceal the errors so that relativelyhigh subjective noise effects may develop.

From the International Publication WO, A, 92 179 48 it is known topositively conceal interfered-with sub-bands or spectral values and thusreduce the subjective noise effect.

SUMMARY OF THE INVENTION

The present invention provides a method of transmitting or storing adigitized encoded audio signal composed of a sequence of informationblocks over interference affected channels, wherein the individualinformation blocks are composed of information units containingdifferent informations, such as control informations, scale factorinformations as well as informations about the encoded sampled valuesderived from a sub-band and/or transformation source coding process,with the information units being associated with respect to encodedsampled values with a certain spectral structure and wherein the encodedaudio signal is subjected to channel coding that is dimensionedaccording to a predetermined error protection, the method including thesteps of effecting an error correction when an error is detected and, ifthe error is not correctable, concealing the error, providing a variablebit error protection for the channel coding of the information unitswith respect to the encoded sampled values, the variable bit errorprotection being a function of the different association of theindividual information units with a predetermined spectral structure sothat information units for the encoded sampled values of lower frequencyaudio signal components have a higher bit error protection thaninformation units for the encoded sampled values of higher frequencyaudio signal components, determining a selected variable errorprotection for the individual information units according to subjectivecriteria as a function of a permissible duration and frequency of errorconcealment measures employed, and containing informations regardingadditional error protection with respect to expanded error detection inat least one unit for the succession of information units regarding theencoded sampled values.

Advantageous modifications of the method according to the invention aredefined in the dependent claims.

The invention is based on the consideration of utilizing the differencesin significance of the information units within each information blockfor a continuously staggered variable error protection in such a waythat interference in the transmission results in defined interferencepatterns which can be easily concealed by suitable techniques. Bypermitting these error patterns, the data rate for transmission can bereduced since fewer data than in the prior art are required forsufficient error protection.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail with reference to thedrawings, in which:

FIG. 1 is a schematic representation of an information block composed ofa sequence of nine information units provided with different errorprotection levels as employed in the method according to the invention;

FIG. 2 is a schematic representation similar to FIG. 1 for analternative embodiment; and

FIG. 3 is a schematic representation of the two channels of astereophonic audio signal that has been channel coded according to themethod of the invention as shown in FIG. 2 after the channel decoding.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is assumed hereinafter that a sub-band method is employed as thesource coding method in which the frequency band of the digitized audiosignal is split into a plurality of sub-bands. Each sub-band is assignedto one of six sub-band groups. The bit assignment and thus the datareduction of the sampled values of each sub-band is effected accordingto psychoacoustic aspects (calculation of masking threshold). For thispurpose, the peak value of a certain number of successive sampled valuesis determined in each sub-band and is retained in quantized form as theso-called "scale factor" and is subjected to channel coding togetherwith the data reduced sampled values and the bit assignment information.In addition to the scale factors, selection information for theabsolutely necessary scale factors may be transmitted in order tofurther reduce the number of scale factors to be transmitted and thusthe quantity of data. The FIG. 1 is a schematic representation of such adata reduced, channel coded audio signal 1 that is composed of aplurality of successive information blocks 10. The information blocks 10preferably all have the same length and, in the illustrated example,each comprise a sequence of nine information units 11 to 19. Accordingto the source coding scheme under consideration, the nine informationunits 11 to 19 contain the following information:

    ______________________________________                                        unit 11      bit assignment information                                       unit 12      scale factor selection information                               unit 13      scale factors                                                    unit 14      sub-band group No. 1                                             unit 15      sub-band group No. 2                                             unit 16      sub-band group No. 3                                             unit 17      sub-band group No. 4                                             unit 18      sub-band group No. 5                                             unit 19      sub-band group No. 6                                             ______________________________________                                    

According to the invention, a varying bit error protection 21 to 29 isassigned to the individual information units 11 to 19 during channelcoding. The bit assignment information 11 and the scale factor selectioninformation 12 require high error protection 21 and 22, respectively,since bit errors within these information units may lead to the completedestruction of the audio signal at the output of the source decoder.Since interfered-with scale factors 13 have an extremely annoyingsubjective effect, they also require a relatively high error protection23. Sub-band groups No. 1 to No. 5, which are illustrated as informationunits 14 to 19, are treated differently with respect to their errorprotection 24 to 29. The sub-band sampled values of the sub-bands of thelower frequency range are contained in information unit 14, those of thehighest frequency range to be transmitted are contained in informationunit 19. Sub-band groups No. 1 to No. 6, represented by informationunits 11 to 19, are assigned the data reduced sub-band sampled valuesaccording to their spectral association. The sub-band groups are thusgiven a different level of error protection 24 to 29 which decreasesfrom the low frequencies to the high frequencies. The continuouslystaggered bit error protection in the individual units at the borders ofeach unit is adapted to the bit error protection of the respectiveadjacent information unit.

In the embodiment of FIG. 2, an additional information unit 40 whichprovides additional error protection 50 for units 14 to 19 with respectto better error detection is introduced between information units 13 and14 in order to be better able to detect errors in units 14 to 19. Anapplication of this better error detection is shown in FIG. 3 for atwo-channel stereophonic audio signal. Three successive informationblocks 110, 120, 130 and 210, 220, 230, respectively, are shown in eachone of the two channels No. 1 and No. 2. As shown in FIG. 1 for block10, each one of these blocks is composed of nine information units. Asindicated by hatching, one or two information units containing sub-bandgroup information are interfered with in each one of blocks 110, 130,220 and 230. This interference can be detected with the aid of theadditional error protection 50 of FIG. 2. As an example for errorconcealment, FIG. 3 indicates that the respective interfered withunit(s) are replaced by information units either

(a) from the adjacent channel where they are present at the same timeand in the same sub-band group; or

(b) from the same channel where they are present in the precedinginformation block in the same sub-band group. These possibilities ofsubstitution or concealment are shown in FIG. 3 by the respectivearrows.

I claim:
 1. A method of transmitting or storing a digitized encodedaudio signal composed of a sequence of information blocks overinterference affected channels, wherein the individual informationblocks are composed of information units containing differentinformations, with the information units being associated with respectto encoded sampled values with a certain spectral structure and whereinthe encoded audio signal is subjected to channel coding that isdimensioned according to a predetermined error protection, the methodcomprising the steps of:effecting an error correction when an error isdetected and, if the error is not correctable, concealing the error;providing a variable bit error protection for the channel coding of theinformation units with respect to the encoded sampled values, thevariable bit error protection being a function of the differentassociation of the individual information units with a predeterminedspectral structure so that information units for the encoded sampledvalues of lower frequency audio signal components have a higher biterror protection than information units for the encoded sampled valuesof higher frequency audio signal components; determining a selectedvariable error protection for the individual information units accordingto subjective criteria as a function of a permissible duration andfrequency of error concealment measures employed; and containinginformations regarding additional error protection with respect toexpanded error detection in at least one unit for the succession ofinformation units regarding the encoded sampled values.
 2. A methodaccording to claim 1, further comprising the step of providing acontinuously staggered bit error protection for the channel coding inindividual information units regarding the encoded sampled values.
 3. Amethod according to claim 2, further comprising the step of adapting thecontinuously staggered bit error protection in the individualinformation units at the borders of each unit to the bit errorprotection of the respective adjacent information unit.